Recently, techniques are being quickly developed that a TFT is formed on a substrate having an insulated surface to constitute an electric circuit. Currently, there are many examples which a TFT is used as a switching element of a liquid crystal display device (liquid crystal panel). An active layer, which is the most important part of a TFT, is formed by a semiconductor thin film. While an amorphous silicon film has been frequently used as the semiconductor thin film, a polysilicon film is becoming the main current according to demands of a TFT having a higher operation speed.
A TFT using a polysilicon film (polysilicon TFT) is classified into a high temperature polysilicon TFT and a low temperature polysilicon TFT depending on the processing temperature. While both of them have been manufactured as commercial products, currently, the high temperature polysilicon TFT having reliability and stable characteristics occupies a wide range of the market.
In case that a high temperature polysilicon film is used as an active layer, the crystallinity of the polysilicon film is generally improved by subjecting a heat treatment at a temperature of about from 800 to 1,000.degree. C. For that reason, a quartz substrate having high heat resistance is used as a substrate. Several millions of TFTs which correspond to several liquid crystal panels are usually formed on one quartz substrate so as to improve the throughput by producing plural devices.
In the case that several millions of TFTs are formed on one large substrate to produce plural devices, it is desired that all the TFTs formed on a substrate have the uniform characteristics and normal operation.
In case that several millions of TFTs are formed on a large substrate, it occurs irregular characteristics and defective operation of the TFT under the present circumstances. As a result of SEM and TEM observations of the defects by the inventors, they have been observed in the active layer, which is a factor of the defective operation of the TFT. FIG. 5B shows these defects of a TEM photograph in a schematic cross sectional diagram.
When the inventors observed the surface of low grade quartz substrate which is marketed in low price by an AFM (atomic force microscope), they found many large holes (concave parts having an average depth D of from 70 to 100 nm) dispersed at random on the surface of the substrate as shown in FIG. 15A. In FIG. 15A, the large holes can be found as black spots. The quartz substrate which is marketed in low price has an Rms of from 1 to 1.5 nm and the density of the holes (concave parts) is larger than 10,000 per square centimeter.
In case that the density of the holes (concave parts) is larger than 10,000 per square centimeter, it is known by the inventors experiment that crystal growth is prevented.
The concave part of the substrate which is coming into question has such a shape shown in FIG. 5A that the width of the upper part of the opening r.sub.2 (opening diameter) is slightly smaller than the width of the inner part, and the radius of curvature R.sub.2 at the opening at the upper part of the concave part of the substrate is small. The cross sectional curve at the opening by AFM observation exhibits a steep gradient. The cross sectional curve used herein is a curve obtained by TEM observation or AFM observation when cut at a plane perpendicular to the surface of the substrate.
The inventors have found that the reason of defects depends upon the shape (size and depth) of the large concave parts caved in the surface of low grade substrate which is marketed in low price.
In the conventional process, a semiconductor thin film 110 is directly formed on the surface of a substrate 100 as shown in FIG. 5A. Accordingly, the concave parts which has the substantially same radius of curvature as the radius of curvature at the opening 190 of the concave part on the surface of the substrate is formed on the surface of the semiconductor thin film at the upper part of the concave part in the surface of the substrate. In the step of crystallization of an amorphous silicon film and the step of heat treatment, which are the subsequent manufacturing steps of a TFT, these concave parts having a small radius of curvature at the opening inhibit the crystallization of an amorphous silicon film.
In the miniature unevenness at the bottom of the concave part of the substrate shown in FIG. 5A, solids incline to be formed in the crystallization step, and the Semiconductor thin film is cut off to occur the defective operation as shown in FIG. 5B. The defect which the semiconductor thin film is cut off is called silicon cutout. As an EDX observation of the solids formed in the concave part to investigate the composition, it is found that they are silicides formed by segregation of catalytic elements.
It is evident as described above that in case that a semiconductor thin film is formed on a low-priced substrate having large holes caving in the surface (concave parts having miniature unevenness on the bottom) to produce a TFT, defects are appeared to cause irregular characteristics of TFT and deterioration of yield.
In case that low grade substrate which is marketed in low price is used, silicon cutout occurs by the concave parts on the substrate, which becomes a factor of defective operation of a TFT.
Although a substrate having a surface that is flattened by a special polishing method (Rms of from 0.4 to 0.6 nm) is also marketed as shown in FIG. 15B, it is expensive and is not industrially suitable for a mass production. The observed area of FIGS. 15A and 15B is 10.times.10 .mu.m.sup.2.
Thus, it is the biggest problem to reduce the defects such as silicon cutout and so, which is a factor of defective operation of a TFT, without using the expensive substrate.